This invention relates to charge indicator circuits for a vehicular battery charging system which indicate the charged state of the battery by sensing the potential difference between the rectified output voltage of the battery charging alternator and the battery, and which is capable of indicating a non-charged state of the battery even when the field winding of the alternator is disconnected.
Battery charging systems with a conventional charge indicator of this type comprise an alternator driven by an internal combustion engine and having output winding and a field winding, a rectifier having first and second rectified output terminals and a grounded rectified terminal and rectifying the A.C. output from the alternator, a voltage regulator connecting the first or second rectified output terminal of the rectifier with the field winding of the alternator, and a battery charged by the output from the first rectified output terminal of the rectifier. A charge indicator lamp is connected between the second rectified output terminal and the battery through a key switch, and a resistor connects the junction between the second rectified output terminal and the lamp with an earth terminal. The indicator lamp is energized when the voltage at the second rectified output terminal is lower than a predetermined level, thus allowing the current from the battery to flow through the serial circuit formed by the key switch, the lamp, and the resistor.
In these conventional arrangements, however, said resistor must be given a low resistance value to give the indicator lamp sufficient brightness, which fact results in the disadvantage that the load on the second rectified output terminal becomes greater. Thus the initial turn-on speed of the generator at which the predetermined output voltage is reached becomes higher, and when the internal combustion engine driving the alternator is left idling after started, the rotational speed of the generator does not reach the initial turn-on speed causing the battery to be left in the non-charged stage. Also, as the resistance of said resistor is of a small value, the internal losses of the alternator due to the power dissipated from the second rectified output terminal are increased, causing the external output available from the alternator to decrease. Further, the power loss at said resistor generates heat therein causing the overall dimension of the arrangement to become greater, the reliability thereof to be reduced, and the costs of manufacture to increase.